Thinning and reducing weight are demand of electronic devices such as portable computers for easy carrying. On the other hand, portable electronic devices of this kind may be affected by a force applied from outside, for example, when being carried out in a bag.
For this reason, there is a demand that the housing of a portable electronic device should have a high stiffness in addition to thinness and lightness in weight. In particular, the exterior cover which covers the display module from the back needs to have a sufficient stiffness since the liquid crystal display panel to be built in the display module is mechanically weak.
For example, Jpn. Pat. Appln. KOKAI Publication No. 2008-234100 discloses that ribs of the so-called honeycomb structure are provided an inner side of a housing. It is known that the honeycomb structure has a high mechanical strength per unit volume. Therefore, by providing a rib of a honeycomb structure, it is possible to form a thin, light and highly stiff housing.
By providing integrating a rib of a honeycomb structure to the exterior cover of a display module as one unit, the exterior cover can be formed thin and light in weight. Further, the exterior cover can be used also as a decorative cover exposing to outside. In this manner, the exterior cover can be made further thin and light in weight. The exterior cover can be manufactured by molding a metal such as magnesium by die-casting.
However, if the exterior cover includes a rib of the honeycomb structure, the exterior cover tightly attaches to the metal mold when molding. Therefore, a large resistance is involved when the exterior cover is removed from the metal mold. When the resistance becomes excessively large, the exterior cover may be deformed while the cover is being removed from the metal mold with a push-out pin as the pin pushes the cover.